When you purchase gold shields or receive them from offers, it gets put into a shared pool which all your characters can use. If you make purchases on a character and it does not have enough gold shields, it will start using shields from your shared pool. What you did was you noticed you had one amount on a character, but then went on another character and started using gold shields from the shared pool. When you logged back in the first character, you noticed your amount went down. This is intended so that when you purchase shields, you can use them on all characters.

I had some issues with the new version of Book of Heros, so, started another character with a new download. I added gold to the account. Then, I was able to recover the old character, and the gold from the new character that I am not playing did not follow through to the old character. Will this show up in a 'pool' when I get low? Do I have to transfer it? I just really want to delete that character and have the gold go to my new one. Is this possible?